Orthogonal Frequency Division Multiplexing (OFDM) is a special form of multi-carrier transmission technique in which a single high rate data stream is divided into multiple low rate data streams. These data streams are then modulated using subcarriers which are orthogonal to each other. In an OFDM transmission system, each subcarrier is attenuated individually under the frequency-selective and fast fading channel. Modulation schemes adapted to the signal to noise ratio (SNR) of the individual subcarriers can be employed to improve the performance and data throughput of an OFDM transmission system. A key task performed in an adaptive OFDM transmission system is channel estimation.